I am having trouble changing my login screen background. I can select the default wallpaper, the Kde-default one, but whenever I select my own custom wallpaper, there is no image, just a solid color. Normally, I would just ignore it, but the thing that bugs me is that this is a fresh install of Kubuntu 12.04 64bit, and the OS before this one, the one which I overwrote, had no trouble using the exact same image as the background.

sounds like a permissions issue, try running system-settings as root: Press ALT+F2 to open the run dialog Enter kdesudo system-settings Change the login screen now and see what happens.
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Dirk Hartzer WaldeckJun 22 '12 at 9:47

kdesudo system-settings does not work, it prompts for a password, but nothing else opens nor do I get any errors. kdesudo kcmshell4 kdm worked though. Thanks for that bug link, makes me feel like KDE4 is some alpha piece of software, in which case it shouldn't be used with Kubuntu 12.04 (or earlier)...
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drdrezJun 22 '12 at 14:24

Wonderful answer! :D Errors When I run kcmshell4 kdm I get the error QImage::scaled: Image is a null image 4 times, but only when I first open it. Switching to another image and applying does not cause the error... Wrong extension ? Nope, dolphin and KDM config can generate a thumbnail just fine. Image is a .jpg file. File permissions This is the reason. The owner and group are both myself, but I set the permissions on it so that it could be read by group and other. After toggling those permissions, each time the background toggles with it, so the issue was file permissions
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drdrezJun 22 '12 at 14:36